Nuisance alarm reductions in a physiological monitor

ABSTRACT

A method and apparatus for controlling alarms in a medical diagnostic apparatus where an alarm is generated when a measured value for a physiological parameter is outside a specified range. The method continuously calculates a baseline value, and establishes dynamic thresholds that are related to and continuously track the baseline value. The method determines the amount of time the measured value is past the dynamic threshold, and the amount by which the threshold is passed. Alarms are triggered based upon a combination of the amount of time and the amount by which the threshold is passed. Preferably, the combination is an integral or some function of an integral.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/080,139, filed Mar. 31, 2008, which is a continuation of U.S.application Ser. No. 11/581,503, filed Oct. 16, 2006, which is acontinuation of U.S. application Ser. No. 10/850,513, filed May 19,2004, now U.S. Pat. No. 7,123,950, which is a continuation of U.S.application Ser. No. 09/910,700, filed Jul. 19, 2001, now U.S. Pat. No.6,754,516.

BACKGROUND OF THE INVENTION

The present invention relates to alarms in medical diagnosticsapparatus, and in particular to improvements in reducing nuisance alarmsfor pulse oximeters.

A typical pulse oximeter measures two physiological parameters, percentoxygen saturation of arterial blood hemoglobin (SpO₂) and pulse rate.For alarm purposes, low and high thresholds are set for both SpO₂ andpulse rate, defining normal ranges within which it is desired tomaintain the patient. For example, with a neonate it might be desiredthat sat should remain between 85 and 95 percent and pulse rate shouldremain between 120 and 170 beats per minute. From the two measuredparameters, typically four alarm types can be generated, low sat, highsat, low rate, and high rate. In some pulse oximeters, an alarm beginsimmediately when either sat or rate goes outside the normal range andthe alarm ends immediately when both sat and rate return within thenormal range. Alarms are typically announced by audible and/or visualindicators. Alarms, which are dependent on the instantaneous excursionsof a measured value outside a range, are commonly referred to asconventional alarms.

Each occurrence in which a measured parameter goes outside the normalrange is referred to as an event. Thus, in a typical pulse oximeter,each event coincides with an alarm, and the alarm duration may beidentical to the event duration. Some of the alarms produced by typicalpulse oximeters are not generally considered to correspond to eventsthat are clinically significant. The exact definition of clinicalsignificance varies depending on the patient and circumstances, but isin general related to the severity and duration of the event ofinterest. For example, a very shallow desaturation might only beconsidered significant if sustained for a relatively long period oftime. Likewise, a desaturation of very brief duration might only beconsidered significant if it falls very deep below the low satthreshold. In addition to clinically insignificant alarms, parametermeasurement error due to noise, signal artifact or bias can also producefalse events and trigger alarms. An alarm that does not correspond to aclinically significant event may be considered a nuisance alarm.

Several approaches are available which attempt to reduce the number ofnuisance alarms. Some of these approaches have either looked at loweringthe alarm threshold or waiting some fixed period of time after thethreshold has been crossed before triggering an alarm. Lowering thethreshold can be problematic because a patient's blood oxygen saturationcan remain indefinitely below the original threshold, but above the newthreshold, and an alarm will never be generated. Delaying alarmgeneration by a fixed amount of time is also problematic due to apotentially serious situation in which a patient's saturation abruptlyfalls to and remains at a very low level, requiring prompt medicalattention.

Another solution to the nuisance alarm problem is described in U.S. Pat.No. 5,865,736, entitled, “METHOD AND APPARATUS FOR NUISANCE ALARMREDUCTIONS,” assigned to the assignee herein. The solution described bythe '736 patent is commercially known as the SatSeconds™ AlarmManagement Technology (“SatSecond”) feature. The SatSecond concept hasbeen incorporated into some of assignee's pulse oximeters, such as themodel N-395 pulse oximeter, for enhanced alarm management. FIG. 1 is agraph illustrating the alarm response according to this known SatSecondapproach. This figure shows a conventional and the SatSeconds alarmmanagement methods. This figure, for illustration purposes shows themethods applied to SpO₂ measurements. As described above and shown inFIG. 1, with conventional alarms, SpO₂ (4) or pulse rate (not shown)readings that fall below a specified fixed lower threshold 6 or above aspecified fixed upper threshold (not shown) trigger an audible orvisible alarm state. With the SatSecond methodology, an alarm state isentered only when the second-by-second accumulated product 2, of timeand the degree to which the SpO₂ (4) exceeds the lower 6 or upper (notshown) specified threshold, equals or exceeds an integrated threshold 8.Both the conventional and SatSecond alarm management methods are equallyapplicable to pulse rate or other physiological measurements.

The motivation for the SatSecond method is to reduce the number ofnuisance alarms in which a measured value such as SpO₂ is beyond analarm threshold, but does not represent a clinically significant event.For example, if a caregiver feels that a desaturation of less than 5points below the lower alarm threshold for less than 5 seconds is notclinically meaningful, but rather constitutes a nuisance, the caregivermay set the SatSecond alarm threshold to “25” (5 points for 5 seconds).Then only a deeper desaturation of longer duration (i.e., a product thatexceeds 25 SatSeconds) will initiate an alarm. In certain pulse oximetermodels manufactured by the assignee herein, the product ofsaturation-below-the-threshold and time are accumulated once per second,and this product is compared to the SatSecond alarm threshold each timeis it calculated. The effect of using the SatSecond alarm managementmethod is to reduce the number of nuisance alarms and to alarm morespecifically in response to events that are clinically meaningful asestablished previously by the caregiver.

A limitation in the use of the each of these prior art methods occurswhen the SpO₂ value (or other measured value) is systematically inerror, as in where there is a high or low bias in the measured value,even if the bias error is relatively small. Using the SatSecond methodas an example, this limitation is illustrated in FIG. 2. The graph 22shows a monitored value of SpO₂ having a bias of a few points highrelative to the true saturation 21. As the desaturation event 25 occurs,the lower alarm threshold 24 is not reached until later in the event, ifat all, and the SpO₂ value dips only slightly below the threshold 24.

Accordingly, the SatSecond value 28 (which corresponds to the area ofthe dark hatched region 26 of the upper curve 22 below the lower alarmthreshold 24) never achieves the necessary level 29 needed to initiatean alarm state. FIG. 2 provides an illustration of a “missed” SatSecondalarm due to a bias in the SpO₂ readings. The erroneously high SpO₂value may interfere with the ability to accurately calculate the propervalue of the SatSecond integral 28. The converse (i.e., false SatSecondalarm) would occur if the SpO₂ readings were too low due to a low bias.Hence, SpO₂ bias affects the reliability of measured values and alarmsbased on those values.

Ideally, the SpO₂ reading will be proper (i.e., unbiased from the trueSaO₂). However, under some circumstances such a bias can and does occur.It is known that bias can be created, for example, by an improperlyplaced sensor that shunts light between the emitter and the detector, orby a sensor that has been applied too tightly, or a by patient withsignificant edema. Additionally, sensor placement variations, as well asother factors introduce bias, such that even instrument specificationsacknowledge the presence of bias. Specifically, the accuracyspecification for pulse oximetry sensors readily allows a bias betweentwo sensors placed on the same patient of 3 sat-points. Under suchcircumstances (i.e., two sensors placed on the same patient), one sensormay indicate an alarm state, while the other does not, resulting inambiguity in not knowing which sensor is providing the more correctreading. Thus, although the SatSecond invention greatly reduces nuisancealarms in pulse oximeter readings, the measurements and hence alarmevents may still be susceptible to bias-induced nuisance alarms.Moreover, the SatSecond improvement is based on a product ofsaturation-below-a-fixed threshold (or above) and time. This fixedthreshold can also be problematic, as is described below.

Alarm thresholds described thus far are based on fixed windows, where awindow is defined by the region between a fixed lower and a fixed upperalarm threshold. The fixed lower and upper threshold values are based ontypical default values used for patients in general, and which may beset by the caregiver irrespective of the current instrument readings.However, the fixed window approach may be problematic for patientshaving, for example, a chronically elevated pulse rate value. Some priorart pulse oximeters manufactured by the assignee herein offered afeature known as “Smart Alarms” to allow caregivers to quickly establishthe lower and upper conventional alarm thresholds by manually pressing abutton on the oximeter unit. The “Smart Alarm” is essentially a fixedrelative threshold based on a current physiological value that is beingmonitored. Using this “Smart Alarm” feature, the conventional alarmthresholds could be established at a preset value above and below thecurrent readings of pulse rate, as opposed to the fixed default valuestypically used for patients in general. Thus if a patient is chronicallyat an elevated pulse rate, a revised fixed threshold relative to thecurrent readings could be easily set to a preset number below thecurrent reading so as not to alarm unnecessarily. While the “SmartAlarm” approach allows for the setting of a new fixed threshold that isrelated to the then current readings, it is still a fixed threshold andhence suffers from the same shortcomings described thus far.

There is therefore a need for improvements in medical diagnosticdevices, and in particular to improvements in both integrated or“product”-type and relative-deviation threshold alarms for pulseoximeters.

SUMMARY OF THE INVENTION

The present invention provide a method and apparatus for controllingalarms in a medical diagnostic apparatus where an alarm is generatedwhen a measured value for a physiological parameter is outside aspecified range. The method continuously calculates a baseline value,and establishes dynamic thresholds that are related to and continuouslytrack the baseline value, and triggers an alarm when a measured valueexceeds the dynamic and continuously tracking threshold. In a preferredembodiment, the method determines the amount of time the measured valueis beyond the dynamic threshold, and the amount by which the thresholdis passed, and triggers an alarms based upon a combination of the amountof time and the amount by which the threshold is passed. Preferably, thecombination is an integral or some function of an integral.

In one aspect directed to saturation alarms on a pulse oximeter, analarm is generated when the measured saturation value falls above orbelow a baseline-tracking dynamically changing upper or lower thresholdrespectively.

In another aspect, the preferred embodiment of this invention calculatesthe integral of the amount by which a measured value of the oxygensaturation exceeds an upper baseline-tracking dynamically determinedthreshold, or falls below a lower baseline-tracking dynamicallydetermined threshold. A saturation alarm is generated when the integralexceeds a predetermined value. Similarly, for a pulse rate alarm on apulse oximeter, the preferred embodiment of this invention calculatesthe integral of the amount by which a measured value of the pulse rateexceeds an upper baseline-tracking dynamically-determined threshold, orfalls below a lower baseline-tracking dynamically-determined threshold,and a pulse rate alarm is generated when the integral exceeds apredetermined value. The relative-threshold-based alarm managementmethod of the present invention may also be combined with a fixedthreshold alarm scheme.

For a further understanding of the nature and advantages of theinvention, reference should be made to the following description takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a graph illustrating prior art conventional and SatSeconds™ AlarmManagement Technology alarm management methods.

FIG. 2 is a graph illustrating a missed SatSecond™ alarm due to SpO₂bias.

FIG. 3 is a diagram of an example pulse oximeter.

FIG. 4 is a graph illustrating the alarm management method according toembodiments of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Embodiments of the present invention relate to increasing thereliability of alarms in medical diagnostic equipment measuring aphysiological parameter by improving reductions in nuisance alarms. Inorder to illustrate the invention, the example of a pulse oximeter withthresholds for blood oxygen saturation (SpO₂) will be described. Inparticular, a low saturation event is described. Alternately, highsaturation, low pulse rate, high pulse rate or other alarm parameterscould be addressed by the present invention. In addition, the inventioncould be used for other types of medical diagnostic equipment.

FIG. 3 illustrates a typical pulse oximeter. FIG. 3 illustrates theoximeter housing which includes a digital display 31, select buttons32-35, alarm status lights 36-39, and adjustment knob 40,synchronization status light 41, LED digital view meter 42, and powerswitch 43. A cable 44 to the sensor 45 is shown with the sensor attachedto a finger 46 on a patient's hand 48.

An alarm in accordance with the embodiment of the present invention canbe either produced audibly through a speaker 49, or produced on one ofthe displays described above. Also shown is a display 50 for providingan indication of motion distorting the signal, which could also generatean alai in condition. The display 50 and/or display 31 are also used toprovide other information to the clinician as is deemed necessary. Thepulse oximeter shown in FIG. 3 is shown for exemplary purposes and isnot meant to limit the embodiments of the present invention. Forexample, the sensor 45 can be replaced by other appropriate sensors foruse at other tissue locations including but not limited to the ear,foot, forehead and nose of adult, infant, neonatal and perinatalpatients.

An example of an electronic circuitry for a pulse oximeter which may beconfigured to incorporate the embodiment of the present invention isprovided as FIG. 2 of U.S. Pat. No. 5,865,736, entitled: “METHOD ANDAPPARATUS FOR NUISANCE ALARM REDUCTIONS,” assigned to the assigneeherein, the disclosure of which is hereby incorporated herein in itsentirety. U.S. Pat. No. 5,865,736 also describes algorithms used tocalculate the integral of the difference between the current saturationand a saturation threshold whenever the current saturation is below thesaturation threshold, as well as any necessary additional logic relatedto resetting and clearing the integral and the alarm.

Oxygen saturation can be estimated using various techniques. In onecommon technique, the photocurrent generated by the photo-detector isconditioned and processed to determine the modulation ratio of the redto infrared signals. This modulation ratio has been observed tocorrelate well to arterial oxygen saturation. The pulse oximeters andsensors are empirically calibrated by measuring the modulation ratioover a range of in vivo measured arterial oxygen saturations (SaO₂) on aset of patients, healthy volunteers, or animals. The observedcorrelation is used in an inverse manner to estimate blood oxygensaturation (SpO₂) based on the measured value of modulation ratios of apatient. The estimation of oxygen saturation using modulation ratios isdescribed in U.S. Pat. No. 5,853,364, entitled “METHOD AND APPARATUS FORESTIMATING PHYSIOLOGICAL PARAMETERS USING MODEL-BASED ADAPTIVEFILTERING”, issued Dec. 29, 1998, and U.S. Pat. No. 4,911,167, entitled“METHOD AND APPARATUS FOR DETECTING OPTICAL PULSES”, issued Mar. 27,1990. The relationship between oxygen saturation and modulation ratio isfurther described in U.S. Pat. No. 5,645,059, entitled “MEDICAL SENSORWITH MODULATED ENCODING SCHEME,” issued Jul. 8, 1997. An electronicprocessor for calculating in vivo blood oxygenation levels using pulsedlight is described in U.S. Pat. No. 5,348,004, entitled “ELECTRONICPROCESSOR FOR PULSE OXIMETER,” issued Sep. 20, 1994, and a displaymonitor for a pulse oximeter is described in U.S. Pat. No. 4,653,498,entitled “PULSE OXIMETER MONITOR,” issued Mar. 31, 1987. All fivepatents are assigned to the assignee of the present invention andincorporated herein by reference.

The brief description of pulse oximeters, and associated electroniccircuitry and algorithms described above serve as a contextual fabricfor describing the alarm management method according to embodiments ofthe present invention, which are described below.

FIG. 4 illustrates the behavior of the alarm management method accordingto embodiments of this invention. In one embodiment of the presentinvention, an alarm is generated when saturation signal 70 falls belowthe baseline tracking saturation threshold 74.

In an alternate embodiment, the alarm management method is based on anintegrated-relative-threshold algorithm. A saturation signal 70 iscompared to a low sat threshold 74. Also illustrated is an integralthreshold 78. An excursion 80 produces an integral value 82 that canexceed the integral threshold 78. The value 82 is a product of theamount of time and the amount by which the measured value of oxygensaturation exceeds the threshold. The alarm management method accordingto embodiments of this invention include dynamically and continuouslycalculating a “baseline” value 72 for the SpO₂ readings, andestablishing a continuously and dynamically tracking set of upper (notshown) and lower alarm threshold 74 that continuously and dynamicallyfollow this baseline. Certain embodiments first calculate a baselinevalue for saturation or other physiological parameter of interest, anddefine the dynamic thresholds by offsetting from this baseline. As theinstantaneous readings of SpO₂ (or other variable) wander beyond thesethresholds, the product 82 of time and extent beyond the threshold iscalculated. The low sat alarm threshold 74 tracks the baseline SpO₂trend 72. The baseline trend 72 is an average of the measured SpO₂signal 70, and which is obtained by low-pass filtering the measured SpO₂signal 70. The area under the curve 76 where the instantaneous SpO₂value drops below the lower threshold 74 is calculated (ΔSatSeconds) andan alarm state is entered when the value 82 of the integral equals orexceeds a user defined integral threshold 78. Alternately, a defaultvalue may be used in lieu of the user-defined threshold 78.

In one embodiment, the baseline value 72, which the upper (not shown)and lower thresholds 74 track, is computed by using a low-pass filter.Alternately, the baseline is calculated using a running median filter.Other alternate methods for calculating a baseline may also be used solong as the methodology results in a more slowly varying value for thebaseline than the instantaneous readings 70. Examples of these alternatemethods are described below.

In one alternate embodiment, an “Infinite Impulse Response” filter isused by continuously updating the baseline value using the most recentreading added to the running computation of the past, as shown in Eqn. 1below:Baseline Value=1/N*SpO ₂+(N−1)/N*Last Baseline Value,  Eqn. 1

where N is a number that results in a “slow” response time (e.g. 15minutes)

In another alternate embodiment, the baseline is tracked by using arunning “Finite Impulse Response” filter, where readings taken over apast several minutes are stored and averaged. These baseline-trackingmethods are examples of tracking algorithms, and are not meant to limitthe embodiments of the present invention, as many methods are availablefor calculating the baseline SpO₂ value.

In the embodiments of the present invention, alarm thresholds aredynamic and determined relative to the tracked baseline. In the priorart integral-based methods referred to and described as the SatSecondconcept, the (integral value) alarm threshold is calculated based oninstantaneous readings wandering beyond fixed thresholds established bydefault values or user-specified upper and lower alarm thresholds. Inthe methods embodied by the present invention, the threshold dynamicallyfollows a dynamically calculated baseline, trending up or down with themeasured value, while the baseline dynamically smoothes out theshort-lived excursions in the SpO₂ signal. In other words, a window isestablished by defining upper and lower threshold values that are offsetfrom the baseline by a specified value above and below the baselinerespectively, thus establishing a relative threshold. In this way, anybias that exists between measured SpO₂ and true SaO₂ has minimal effecton the reliability of saturation alarms.

In other embodiments, the dynamic alarm threshold is an offset of acontinuously updated baseline, so that the alarm threshold is directlycomputed in one step, as opposed to calculating a baseline in a firststep and then offsetting the baseline to determine the threshold in asecond step. This is achieved by offsetting a slowly varying average ofthe measured value by a certain amount above and below the measuredvalue to define upper and lower relative thresholds respectively.

The improved alarm management methodology, as embodied by the presentinvention can be used independently, or in conjunction with a fixedwindow method, with the combined alarm thresholds chosen to complementone another. The following examples, described below demonstrate theutility of the improvements as embodied by the present invention.

Examples of SpO₂ Monitoring Scenarios

The following assumptions apply to each of the following examples:

-   -   The baseline true value of SaO₂ is 95%,    -   The fixed alarm threshold is set to 85% and the SatSecond (SS)        value needed to trigger an alarm is set to 25 sat-second,    -   the ΔSatSecond (ASS), relative-threshold is set to 25 based on a        threshold of 10% less than a running baseline,    -   integrated products of deviation-from-threshold times time are        calculated once every second.

Thus the thresholds are nominally equal (i.e., a drop in sat of 10%),but the ΔSS alarm triggers based on the change from a baseline, whilethe SS alarm triggers based on crossing the fixed value of 85% SpO₂(i.e., 10% drop from the 95% true baseline value).

EXAMPLE 1 Correct SpO2 Readings

This example involves a scenario where the SpO₂ readings are correct, orin other words, the SpO₂ and SaO₂ readings are equivalent, since nomeasurement bias is present. In this example, if the SaO₂ value drops 2points below the fixed threshold (83% SaO₂ and SpO₂), the SS alarm willsounds in 13 seconds (13 sec*2 sat deviation=26 sat-seconds, which isgreater than 25 sat seconds). The ΔSS level triggers at an equivalentpoint, but is redundant. Both trigger events represent True Positives(TP). A Positive event is an event where the diagnostic device triggersan alarm. A “True” condition refers to the real and actual datasupporting the presence of an alarm condition. Thus a TP event is wherethe diagnostic device senses an event and triggers an alarm where a realclinically significant event was present. A TP event represents an eventwhere the diagnostic device has correctly identified a clinicallysignificant event and triggered an alarm.

If the SaO₂ drops to 2 points above the fixed threshold (87%), neitheralarm will sound as SpO₂ does not cross either of the thresholds. Bothnon-trigger events will then represent True Negatives (TN). A Negativeevent is an event where the diagnostic device does not trigger an alarm.Thus a TN event is where the diagnostic device does not and should nottrigger an alarm. A TN event represents an event where the diagnosticdevice has correctly identified a non-existent or clinicallyinsignificant event and does not trigger an alarm.

EXAMPLE 2 Positively Biased SpO₂ Readings

This example involves a scenario where the SpO₂ baseline reads 98%, 3points high relative to the true SaO₂ value due to a reading withpositive bias. In this example, if the SaO₂ value drops 12 points, thatis 2 points below the threshold (83%), an alarm state should occur in 13seconds (13 sec*2 sat deviation=26 sat-seconds, which is greater than 25sat seconds), but does not due to the bias resulting in a SpO₂ readingof 86%. This results in a False Negative (FN) for the SatSecond (SS)threshold. A “False” event refers to a state sensed by the diagnosticdevice that is not supported by the real and actual data. Thus a FNevent is where a diagnostic device should trigger an alarm but does not.A FN event represents an event where the diagnostic device has missed aclinically significant event and not triggered an alarm. In thisexample, the SS alarm would never occur since the SpO₂ value never dropsbelow 85%. Further, a conventional SpO₂ set to less than 85% would alsomiss this event.

Since SpO₂ drops by 12 points, this will result in an ΔSS alarm in 13seconds (2 points below the dynamic alarm threshold for 13 seconds=26ΔSS). This event will then be a True Positive for ASS. This exampleclearly points out the improvement provided by the relative sat-secondmethod over the fixed sat-second method for a case where themeasurements are positively biased, since the fixed threshold alarmwould miss the event, but a relative and dynamic alarm threshold wouldcapture the event.

If SaO₂ drops 8 points, to two points above the threshold (87%), SpO₂readings become 90% (98%-8%) and no SS alarm would sound, thus resultingin a True Negative event. The same result occurs with ΔSS, as the8-point drop isn't sufficient to trigger the ΔSS integral. Recall thatthe ΔSS relative-threshold is set to 25 based on a threshold of 10% lessthan a running baseline.

EXAMPLE 3 Negatively Biased SpO₂ Readings

This example involves a scenario where the SpO₂ baseline reads 92%, 3points low relative to the true SaO₂ value due to a negatively biasedreading. In this example, if the SaO₂ drops 12 points, 2 points belowthe fixed threshold (83%), with the SpO₂ reading 80% due to the negativebias, the SS alarm is triggered after 5 seconds (5 points belowthreshold* seconds=25 SS). The ΔSS alarm will trigger in 13 seconds (2points below the 10 point allowable threshold takes 13 seconds to exceed25 Δsat-seconds). Thus this scenario results in a TP for both alarmmethods, though a little sooner than required for the fixed SS alarm.

If the SaO₂ drops 8 points, 2 points above the fixed threshold (87%),the SS alarm should never engage, but it does trigger a FP in 25 secondsdue to the 3-point low bias (SpO₂=84%). The SpO₂ drop from 92% to 84%does not trigger an ΔSS alarm since it does not exceed the 10% necessarythreshold drop. Here the advantage of the improved alarm management isillustrated since this clinically insignificant event (by definition)would trigger a FP SS alarm, and hence create an unnecessary or anuisance alarm, while the dynamic threshold design (ΔSS) registers a TN.

As can be seen from these examples, the sensitivity and specificity forthe dynamic and continuous baseline tracking approach is improved in thepresence of bias over a fixed threshold approach. Particularly, therelative-dynamic threshold method as embodied in this invention isespecially adept at capturing clinically significant events in caseswhere the diagnostic device's readings are positively biased. When nobias is present, both the dynamic and fixed threshold approaches areequivalent in their sensitivity.

Alternate embodiments of this invention combine both the dynamicrelative threshold methods as embodied by this invention and the knownfixed threshold methods. This combined embodiment is especially usefulwhere the diagnostic device is configured to prevent a slowly decayingbaseline SaO₂ (and thus SpO₂) from falsely missing hypoxia. In such anembodiment, the fixed threshold is set at a lower value so as to avoidfalse positives, however, the lower fixed threshold is judiciously setto catch a potentially slowly deteriorating patient condition. Thisarrangement is useful because a dynamic and relative baseline trackingalarm management scheme would also slowly track the decaying baselineand thus not trigger a low saturation alarm.

As will be understood by those of skill in the art, the presentinvention which is related to calculating an integral of the time anddepth product of a monitored variable, using a dynamically trackingthreshold for initiating and calculating an integral, may be embodied inother specific forms without departing from the essentialcharacteristics thereof. For example, variables other than SpO₂ such aspulse rate, blood pressure, temperature, or any other physiologicalvariable could be continuously or periodically tracked. Accordingly, theforegoing disclosure is intended to be illustrative, but not limiting,of the scope of the invention, which is set forth in the followingclaims.

What is claimed is:
 1. A medical diagnostic apparatus, comprising: asensor configured to deliver a signal based at least in part on aphysiological parameter; and a processor configured to determinemeasurements of the physiological parameter based at least in part onthe signal, to establish at least one dynamic alarm threshold based on adynamically calculated baseline of the measurements, and to trigger analarm when the measurements exceed the dynamic alarm threshold.
 2. Theapparatus of claim 1, wherein the processor is configured to establish adynamic high alarm threshold and a dynamic low alarm threshold based onthe dynamically calculated baseline of the measurements.
 3. Theapparatus of claim 1, wherein the processor is configured to establishthe at least one dynamic alarm threshold by calculating a dynamicbaseline value and adding a specified value to or subtracting aspecified value from the dynamic baseline value.
 4. The apparatus ofclaim 1, wherein the processor is configured to establish the at leastone alarm threshold by offsetting the dynamically calculated baseline.5. The apparatus of claim 1, wherein the physiological parametercomprises at least one of a pulse rate measurement, an oxygen saturationmeasurement, a blood pressure measurement, and a temperaturemeasurement.
 6. A method of operating a medical device, comprising:acquiring measurements of a physiological parameter; establishing atleast one dynamic alarm threshold based on a dynamically calculatedbaseline of the measurements; determining an amount of time themeasurements are beyond the dynamic alarm threshold and an amount bywhich the measurements pass the dynamic alarm threshold; and triggeringan alarm based on a combination of the amount of time and the amount bywhich the dynamic alarm threshold is passed.
 7. The method of claim 6,comprising establishing a dynamic high alarm threshold and a dynamic lowalarm threshold based on the dynamically calculated baseline of themeasurements.
 8. The method of claim 6, wherein establishing the atleast one dynamic alarm threshold comprises at least one of adding aspecified value to or subtracting a specified value from the dynamicbaseline.
 9. The method of claim 6, wherein establishing the at leastone dynamic alarm threshold comprises calculating an offset dynamicbaseline.
 10. The method of claim 6, wherein the combination of theamount of time and the amount by which the dynamic alarm threshold ispassed comprises an integral or a function of an integral.
 11. Themethod of claim 6, wherein the measurements of the physiologicalparameter comprise at least one of a pulse rate measurement, an oxygensaturation measurement, a blood pressure measurement, and a temperaturemeasurement.
 12. The method of claim 6, wherein triggering the alarmcomprises at least one of sounding an audible alarm or displaying avisual alarm.
 13. A patient monitoring system, comprising: at least onechannel configured to acquire measurements of a physiological parameter;and a processor configured to determine a dynamic baseline of themeasurements, to establish at least one dynamic alarm threshold based onthe dynamic baseline, to determine an amount of time the measurementsare beyond the dynamic alarm threshold and an amount by which themeasurements pass the dynamic alarm threshold, and to trigger an alarmbased on a combination of the amount of time and the amount by which thedynamic alarm threshold is passed.
 14. The system of claim 13, whereinthe combination of the amount of time and the amount by which thedynamic alarm threshold is passed comprises an integral or a function ofan integral.
 15. The system of claim 14, wherein the integral comprisesan integral of a difference between a current measurement and thedynamic alarm threshold when the current measurement exceeds the dynamicalarm threshold.
 16. The system of claim 13, wherein the processor isconfigured to determine the dynamic baseline of the measurements bycalculating a baseline value that varies slower than the measurements.17. The system of claim 13, wherein the measurements of thephysiological parameter comprise at least one of a pulse ratemeasurement, an oxygen saturation measurement, a blood pressuremeasurement, and a temperature measurement.
 18. The system of claim 13,wherein the processor is configured to determine the dynamic baseline ofthe measurements by low-pass filtering, running median filtering,infinite impulse response filtering, or finite impulse responsefiltering.
 19. The system of claim 13, wherein the processor isconfigured to establish the at least one dynamic alarm threshold byadding a specified value to or subtracting a specified value from thedynamic baseline.
 20. The system of claim 13, wherein the processor isconfigured to trigger the alarm by sounding an audible alarm, displayinga visual alarm, or both.